Fast Tool Servo (FTS) technology can enable precise positioning or manufacturing of complicated sculptured surfaces having nanometer-scale resolution requirements. Such surfaces are used in a wide range of products, including films for brightness enhancement and controlled reflectivity, sine wave ring mirrors used in carbon dioxide (CO2) laser resonators, molds for contact lenses, as well as in micro-optical devices such as Fresnel lenses, multi-focal lenses and microlens arrays. The limits on stroke, bandwidth, acceleration, and position noise of the FTS impose limits on the types, quality, and rate at which the intended surfaces can be produced. The requirements for obtaining high throughput for an FTS include high bandwidth, high acceleration, and high accuracy.
FTS actuators can be categorized as four types: piezoelectric actuators, magnetostrictive actuators, Lorentz force motors (including linear and rotary) and variable reluctance actuators. According to moving strokes, short stroke can be defined as being less than 100 μm, intermediate as being between 100 μm and 1 mm and long stroke as being above 1 mm.
Most of the high bandwidth, short stroke FTS's are based on piezoelectric actuators. Piezoelectric FTS's have the advantage that they can readily achieve bandwidths on the order of several kHz and high acceleration on the order of hundreds of G's, are capable of nanometer resolution of positioning, and can achieve high stiffness (usually greater than 50 N/μm in the typical sizes used).
However, piezoelectrically actuated FTS's also have significant disadvantages. When the piezo materials undergo deformation, heat is generated by hysteresis loss, especially in high bandwidth and high acceleration applications. In addition, it may be difficult to couple the piezoelectric material to a moving payload in such a way as to not introduce parasitic strains in the actuator. Furthermore, piezoelectric FTS's require large and expensive high-voltage, high current amplifiers to drive these devices. Still another shortcoming associated with piezoelectrically activated FTS's is that the structural resonance modes of the PZT stacks limit working frequency ranges, because operation near the internal resonances can cause local tensile failure of PZT ceramics. Piezoelectric actuators can also be used in other high-bandwidth, short-stroke applications such as electric engraving, mirror positioning and scanning and micropositioning, but have the same disadvantages.
However, electromagnetic actuators do not have such problems and thus are a promising alternative. Variable reluctance actuators in FTS's have not been developed extensively, because of the difficulty of controlling these devices in the presence of the inherent non-linearities. There still remains a need for developing an electromagnetically driven actuator as a replacement for widely used piezoelectrically actuated systems.